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Add debian 9 based image for aarch64/arm64 cross compile builds

See merge request !10

Add deploy-to-obs job (osc)

See merge request !9

Add `openssh` to the AUR deploy image (for cloning over ssh)

See merge request !8

Add a deploy image for AUR packages

See merge request !7

Pin the `clang-format` version to `9`

See merge request !6

Pin the `clang-format` version to `9` to be compatible with the macOS
`/usr/local`. Use `update-alternatives` to set `clang-format-9` as the
default `clang-format` binary.

Pin the `clang-format` version to `8`

See merge request !5

Pin the `cmake-format` version to `0.6.7`

See merge request !4

This commit belongs to the changes of
[GR MR !339](https://iffgit.fz-juelich.de/Scientific-IT-Systems/gr/merge_requests/339).

Add `six` for `cmake_format`

See merge request !3

`cmake_format` needs `six` but it is not declared as a dependency.
Therefore, install `python3-six` explicitly.

Add missing `file` command to the code-style-check image

See merge request !2

The last commit added the `no-install-recommends` option to apt to
create a smaller image but the needed `file` command was removed. This
commit adds `file` again.

Add `cmake_format` to the code style check Dockerfile

See merge request !1

These changes are needed for
[MR !317](https://iffgit.fz-juelich.de/Scientific-IT-Systems/gr/merge_requests/317).